As a general rule the approach of a tool member towards the workpiece in machine tools is brought about with a relatively high speed in order to reduce as much as possible the dead time of the machine. However, when the tool member comes into contact with the workpiece, a very rapid reduction of this high advancing speed is desirable in order to avoid deterioration, as much to the tool member as to the workpiece.
Arrangements have long been known employed particularly in grinding machines, which include an acoustic sensor preferably of the piezo-electric type, intended to detect an acoustic emission which is generated when the grinding wheel touches the workpiece, the signal thus obtained being employed following amplification to reduce, if necessary to zero, the relatively high speed employed during the approach phase.
Such an arrangement may likewise be usefully employed to stop the machine in case of accidental collision between the tool member and the workpiece. A collision may be brought about for instance by a faulty setting of the workpiece which may bring about serious damage and injuries to the operator.
Such an arrangement is described for instance in the Swiss Pat. No. 585.609 where the sensor is fastened to the workpiece bearing headstock forming a part of the machine framework. Arrangements are likewise known where the sensor is fastened to the grinding wheel headstock.
At the same time such known arrangements present a difficulty in that the inherent noise generated by the tool when it comes into contact with the workpiece is scarcely different in intensity and frequency from the stray noises produced by the machine, for instance noises coming from the bearings, gears, pneumatic or hydraulic driving means, etc., to the extent that proper operation of the arrangement may not always be assured.
In the case where the sensor is fixed to the workpiece headstock, the useful signal, i.e. that which is produced when the grinding wheel comes into contact with the workpiece, must pass through bearings supporting the workpiece spindle before reaching the sensor, this representing a substantial resistance to the transmission of vibrations produced at the moment of contact. In the same manner, in cases where the sensor is fixed to the grinding wheel headstock, the useful noise must pass through bearings supporting the grinder spindle which in themselves, generate substantial vibrations due above all to the high rotation speed of the grinding wheel.
If, in certain specific cases of machining adapted above all to machines for machining exterior diameters of workpieces, the arrangements described hereinabove have proved most useful and operate to full satisfaction, the problem becomes more complex when it is one of grinding interior diameters of workpieces. In this latter case, in effect, the useful signal to be detected exhibits a greatly reduced amplitude due above all to the small dimensions of the grinding wheel as well as to the small force exerted by such grinding wheel against the workpiece. It will thus be difficult to obtain a sufficient distinction between the useful signal and the stray signal brought about by the machine, the first being often drowned in the second.